A number of atmospheric pressure ionization (API) sources have been developed for producing ions from a sample at atmospheric pressure. One well-known and important example is the electrospray ionization (ESI) source. The electrospray ionization technique, and more specifically electrospray ionization sources interfaced to mass spectrometers, has opened a new era of study for the molecular weight determination of labile and involatile biological compounds. In electrospray ionization, singly or multiply charged ions in the gas phase are produced from a solution at atmospheric pressure. The mass-to-charge (m/z) ratio of the ions that are produced by electrospray ionization depends on the molecular weight of the analyte and the solution chemistry conditions. Fenn et al. in U.S. Pat. No. 5,130,538 describes extensively the production of singly and multiply charged ions by electrospray ionization at atmospheric pressure.
Briefly, the electrospray process consists of flowing a sample liquid through a small tube or needle, which is maintained at a high voltage relative to a nearby surface. The voltage gradient at the tip of the needle causes the liquid to be dispersed into fine electrically charged droplets. Under appropriate conditions the electrospray resembles a symmetrical cone consisting of a very fine mist of droplets of ca. 1 μm in diameter. Excellent sensitivity and ion current stability is obtained if a fine mist is produced. Unfortunately, the electrospray “quality” is highly dependent on the bulk properties of the solution that is being analyzed, such as for instance surface tension and conductivity. The ionization mechanism involves desorption at atmospheric pressure of ions from the fine electrically charged particles. In many cases a heated gas is flowed to enhance desolvation of the electrosprayed droplets. The ions created by the electrospray process are then mass analyzed using a mass analyzer.
In electrospray ionization the ions are formed in an ionizing region, which is generally maintained at atmospheric pressure, and are drawn through an orifice or ion transfer tube into a low-pressure region where they undergo a free jet expansion. U.S. Pat. No. 4,542,293 describes the use of an ion transfer tube for conducting ions between the ionizing electrospray region at atmospheric pressure and a low-pressure region. A glass, metal or quartz capillary is suitable for this purpose. Ions and gas are caused to flow from the ionization region through the ion transfer tube into the low-pressure region where the free jet expansion occurs. A conducting skimmer is disposed adjacent the end of the tube and is maintained in a field which causes further acceleration of the ions through a skimmer orifice and into a lower pressure region including focusing lenses and analyzing apparatus. Alternatively, the skimmer can be maintained at ground. The skimmer orifice samples a portion of the gas expanding in the free jet, effectively serving to separate the higher-pressure viscous gas flow of the free jet that is found in the first vacuum pumping stage from subsequent vacuum pumping stages, which are maintained at lower background pressure relative to the first pumping stage. Once ions pass through the skimmer orifice, they may be required to pass through one or more additional pumping stages before entering the mass analyzer.
For practical and cost reasons, limited pumping speeds are employed in mass spectrometer instrumentation. Consequently, only a small amount of the ion laden atmospheric pressure gas is “leaked” into vacuum through the ion transfer tube. This has the unfortunate effect of limiting the sensitivity of the mass spectrometer, thereby requiring higher concentration of analyte in the sample solution for detection by the mass spectrometer. One way to augment the sensitivity of a mass spectrometer, such that lower concentrations of analyte can be detected, is to increase the amount of analyte containing vapor that is transferred from the ionization region into the vacuum region of the mass spectrometer. This is accomplished by increasing the throughput of the ion transfer tube, either by increasing the tube diameter or by reducing the tube length. Unfortunately, the resulting increased gas load causes the pressures in the vacuum chambers to increase as well. Since it is necessary to maintain the mass analyzer and detector region under high vacuum conditions, the increase in pressure must be counteracted by increasing the number of vacuum pumps employed and/or increasing the pumping capacity of the vacuum pumps. Of course, increasing the number and/or capacity of the vacuum pumps also increases the cost of the mass spectrometer, as well as the power requirements, shipping weight and cost, and bench space requirements.
There is a need for a system that increases the throughput of the ion transfer tube interface and that does not require additional vacuum pumps or increased pumping capacity of the vacuum pumps.